It is known in the art relating to diesel engines to provide electrically heated glow plugs in the combustion or precombustion chambers where fuel is injected to aid in igniting the fuel during starting and cold engine operation. Many types of such glow plugs have been utilized, each having its attendant operational characteristics.
Simple constant resistance heaters when used without additional controls were subject to the objection of excessive warm-up time, often more than one minute, before the glow plugs reached an operating temperature adequate to permit engine starting. This waiting time has been greatly reduced by present systems combining fast heating glow plugs with control devices that interrupt or modulate current flow to maintain the operating temperature in a desired range once it has been reached. Such systems operate effectively but are subject to the objection of added cost and complexity.
Some engine makers have favored self regulating type glow plugs whose heating coils are made of material having a positive temperature coefficient of electrical resistance (PTC) that allows somewhat faster warm-up while limiting the ultimate operating temperature through increasing resistance of the coil with increasing temperature. Dual coil glow plugs have also been developed in which a heating coil at the tip is connected in series with a PTC coil in the heater body to provide somewhat improved performance. However these systems have usually compromised the rapid warm-up capabilities and/or the glow plug durability characteristics of the fast heat control systems.
Our studies of prior PTC and dual coil glow plug designs have found that durability problems have resulted in part from the limited temperature capability of the PTC material, which is subject to oxidation at the operating temperatures required to obtain satisfactory starting of prechamber type indirect injection diesel engines. In some dual coil designs excessive temperatures of the PTC coil have arisen from physical adjacency to the high temperature heater or glow coil in the tip as well as from the selection of wire sizes and materials made to promote fast heating of the glow plug. Typically such designs must be shut off within a few seconds after they reach operating temperature in order to avoid operation at excessive temperatures which would seriously impair their life. Also, close connection of the coils causes early regulation by the PTC coil that reduces current flow too soon and delays warm-up of the heater coil to its desired operating temperature. Another problem we have discovered in some dual coil designs is connection of the coils through adjacent single wires with a very small weld and in a manner that causes high resistance and locally high operating temperatures leading to early failure.